1. Field of the Invention
This invention is directed to novel star-branched elastomeric block copolymers having unsaturation only on the blocks at the free end, (i.e. the uncoupled end) of each branch. More particularly, the invention is directed to solid elastomeric star-branched block copolymers comprising branches wherein an interior block of each branch is substantially selectively hydrogenated and therefore contains substantially no unsaturated groups, while each of the terminal blocks of each branch contains a sufficient amount of unsaturation for curing the block copolymers.
The invention is additionally directed to chemically modified derivatives of the above star-branched block copolymers.
Crosslinking of the polymers of the invention produces vulcanizates having unusual properties, e.g., high elongation and excellent aging characteristics.
2. Description of Related Art
Elastomers (or rubbers) of either natural or synthetic origin usually require vulcanization for transformation into insoluble, non-deformable high strength elastomeric products. Before vulcanization, rubbers possess inferior properties and low strength which limit their utility.
There are a number of well known methods for achieving the vulcanization, also referred to as crosslinking, of unsaturated elastomers. Such methods include the use of sulfur and accelerators, peroxides, benzoquinone dioxime, certain phenolic resins and similar agents. Any of the above or any other well known vulcanizing techniques may be utilized to crosslink the elastomers of this invention.
The great majority of currently known synthetic elastomers are based on polymers or copolymers of butadiene or isoprene. These polymers, which include cis-polybutadiene, emulsion polybutadiene (EBR), styrene-butadiene copolymer (SBR), butadiene-acrylonitrile copolymer (NBR) and cis-polyisoprene, provide raw materials for the production of a very large volume of rubber goods, such as automotive tires, conveyor belts, adhesives, footwear, sponge and mechanical goods. Because of the high degree of unsaturation inherent in the polymeric backbones, these elastomers are easily and quickly vulcanizable alone or in blends. A secondary consequence of the high degree of backbone unsaturation is the instability of such elastomers in the presence of ozone and oxygen, both of which promote rapid deterioration of these elastomers.
Butyl rubber, which is a copolymer of isobutylene and 2-3% by weight (wt.) of isoprene, represents a class of elastomers far more resistant to oxygen and ozone than those based on butadiene or isoprene. The backbone of butyl rubber is primarily polyisobutylene (which provides a saturated spine) into which there is randomly copolymerized about 2-3% by wt. of isoprene to provide unsaturated sites for vulcanization. Butyl rubber finds limited use because of its relatively poor elastomeric properties, and is used primarily in applications which take advantage of its damping properties, weathering resistance and low gas permeability.
Ethylene-propylene-diene rubber (EPDM) has enjoyed substantial commercial growth as a synthetic rubber since it combines excellent oxidation resistance with good elastomeric properties. This elastomer is prepared by the polymerization of ethylene, propylene and a non-conjugated diene, such as 1,4-hexadiene, dicyclopentadiene or ethylidene norbornene. Diene incorporation is typically 5-10% by weight (wt.). The diene is randomly incorporated into the saturated ethylene-propylene backbone to provide pendant vulcanization sites.
The above prior art elastomers, with either high or low levels of unsaturation, are characterized in that, having random unsaturation, they are randomly crosslinked during vulcanization. The success of vulcanization in incorporating all molecular chains into the final crosslinked network with minimal "loose ends" is termed the degree of network perfection. An imperfect network, wherein crosslinks occur randomly and sometimes not near the end of a molecular chain, produces a vulcanized polymer having poor mechanical and elastomeric properties caused by chain ends which are not a part of the tightly bound network. In order to insure the highest degree of network perfection attainable, randomly unsaturated elastomers must be crosslinked extensively. The large number of crosslinks necessary (25 to 40 per 100,000 molecular weight) dictates that the average distance between crosslinks (M.sub.c) must be relatively small in comparison with the dimensions of the whole molecule. Elastomeric properties, such as elongation, depend greatly on M.sub.c --the smaller the M.sub.c the worse are the elastomeric properties, e.g., the lower the elongation of the vulcanized polymer.
Various block copolymers having excellent elastomeric properties, especially elongation, have been made in the past. For example, a block copolymer commonly known as KRATON, manufactured by Shell Chemical Company, which has outstanding properties at room temperature, is a thermoplastic elastomer consisting of block segments of polymerized styrene units and polymerized aliphatic diolefin units, such as butadiene or isoprene. The most common structure of KRATON is the linear A-B-A block, such as styrene-butadiene-styrene (S-B-S) or styrene-isoprene-styrene (S-I-S). One of such rubbers is believed to be described by Jones, U.S. Pat. No. 3,431,323. Jones discloses block copolymers containing block segments of polymerized vinyl arene monomer units, e.g., styrene, butadiene monomer units, and vinyl arene units. After the block copolymer is prepared, it may be subjected to hydrogenation to such a degree that the unsaturation of the polybutadiene block is reduced to less than 10% of its original value, while 10-25% of the poly-vinyl arene block segments are hydrogenated. Although the KRATON triblock copolymers have excellent elastomeric properties at room temperature, since they are thermoplastic materials they lose these properties at temperatures of about 80.degree. C. (and higher). In addition, since these polymers are not chemically crosslinked, they are soluble in many organic solvents. These latter two deficiencies place some restrictions on the viable areas of application for these polymers.
Falk, JOURNAL OF POLYMER SCIENCE: PART A-1, Volume 9, 2617-2623 (1971), the entire contents of which are incorporated herein by reference, discloses a method of selectively hydrogenating 1,4-polybutadiene units in the presence of 1,4-polyisoprene units. More particularly, Falk discloses selective hydrogenation of the 1,4-polybutadiene block segment in the block copolymer of 1,4-polybutadiene--1,4-polyisoprene--1,4polybutadiene and in random copolymers of butadiene and isoprene, with both polymerized monomers having predominately 1,4-microstructure. Selective hydrogenation is conducted in the presence of hydrogen and a catalyst made by the reaction of organoaluminum or lithium compounds with transition metal salts of 2-ethylhexanoic acid.
Falk, DIE ANGEWANDTE CHEMIE 21 (1972) 17-23 (No. 286), the entire contents of which are also incorporated herein by reference, discloses the selective hydrogenation of 1,4polybutadiene segments in a block copolymer of 1,4-polybutadiene-1,4-polyisoprene-1,4-polybutadiene.
Hoxmeier, Published European Patent Application 88202449.0, filed on Nov. 2, 1988, Publication Number 0 315 280, published on May 10, 1989, and corresponding U.S. Pat. No. 4,879,349, disclose a method of selectively hydrogenating a polymer made from at least two different conjugated diolefins. One of the two diolefins is more substituted in the 2,3 and/or 4 carbon atoms than the other diolefin and produces tri- or tetra-substituted double bonds after polymerization. The selective hydrogenation is conducted under such conditions as to hydrogenate the ethylenic unsaturation incorporated into the polymer from the lesser substituted conjugated diolefin, while leaving unsaturated at least a portion of the tri- or tetra-ethylenic unsaturation incorporated into the polymer by the more substituted conjugated diolefin.
Mohajer et al., Hydrogenated linear block copolymers of butadiene and isoprene: effects of variation of composition and sequence architecture on properties, 23 POLYMER 1523-1535 (September 1982) disclose essentially completely hydrogenated butadiene-isoprene-butadiene (HBIB), HIBI and HBI block copolymers in which butadiene has a predominantly 1, 4-microstructure.
Kuraray K K, Japanese published patent application Number JP-328729, filed on Dec. 12, 1987, published on Jul. 4, 1989, discloses a resin composition comprising 70-99% wt. of a polyolefin (preferably polyethylene or polypropylene) and 1-30% wt. of a copolymer obtained by hydrogenation of at least 50% of the unsaturated bonds of an isoprene/butadiene copolymer.
As exemplified by the foregoing references, the prior art has failed to produce a polymer having a saturated backbone for oxidation stability which has unsaturated bonds only on the ends of block polymer chains. Such a block polymer could be vulcanized or selectively functionalized at the terminal ends thereof. The functionalization would expand the utility of the polymer.